1. Field of the Invention
This invention relates to a (meth)acrylate, a polymer, a photoresist composition, and a pattern forming process making use of the composition. More specifically, the present invention is concerned with a photoresist composition and a pattern forming process, which are suitable for use in a lithographic step in the fabrication of a semiconductor device, especially in lithography making use of radiation of 220 nm or shorter in wavelength as exposure radiation.
2. Description of the Related Art
In the field of fabrication of various electronic devices requiring small geometry processing of the half micron order and led by semiconductor devices, there is an ever-increasing demand toward devices of still higher density and integration. This has led to still severer requirements for lithographic technology which is adopted for the formation of submicrometer patterns.
As one of measures for achieving miniaturization of a pattern, there is an approach to shorten the wavelength of exposure radiation which is used upon formation of a resist pattern. For a mass-fabrication process of 256 Mb DRAMs (processing dimension: &lt;0.25 .mu.m), use of KrF excimer laser (wavelength: 248 nm) of shorter wavelength than i-line (wavelength: 365 nm) as an exposure radiation source in place of i-line is positively considered these days.
However, a radiation source of a still shorter wavelength is considered to be needed for the fabrication of DRAMs having an integration degree of 1 Gb or higher which requires still smaller geometry processing technology (processing dimension: &lt;0.18 .mu.m). In particular, use of photolithography making use of ArF excimer laser (wavelength: 193 nm) has been reported recently (Donald C. Hoffer, et al., Journal of Photopolymer Science and Technology, 9(3), 387-397 (1996).
There is accordingly an outstanding desire for the development of a resist which can be successfully employed in photolithography making use of ArF radiation. Such an ArF exposure resist is required to achieve an improvement in the cost performance of laser because, inter alia, the gas life of the excimer laser light source is short and a laser apparatus itself is expensive.
In addition to such a high resolution as permitting a still further reduction in the processing dimension, the ArF resist is also required to exhibit still higher sensitivity. As a method for providing a resist with higher sensitivity, chemical amplification making use of a photoacid generator as a sensitizer is known widely. As a representative example, JP Kokoku 2-27660 discloses a resist which is composed, in combination, of triphenylsulfonium hexafluoroarsenate and poly(p-tert-butoxycarbonyloxy-.alpha.-methylstyrene). Chemical modification resists are now extensively employed as KrF eximer laser resists [for example, Hiroshi Ito, C. Grantwillson, American Chemical Society Symposium Series 242, 11-23 (1984)]. A chemically amplified resist is characterized in that protonic acid, which is generated from a photoacid generator as a component of the resist upon exposure to radiation, undergoes an acid-catalyzed reaction with a resist resin or the like when subjected to a heat treatment after the exposure. As a result, chemically amplified resists have attained far higher sensitivity over conventional resists the photoreaction efficiencies (reactions per photon) of which are lower than 1. Nowadays, most of newly developed resists are of the chemically amplified type, and the adoption of a chemical amplification mechanism has become indispensable in the development of a high-sensitivity material which can meet the trend toward an exposure radiation source of shorter wavelength.
In lithography making use of radiation of a short wavelength of 220 nm or shorter led by ArF eximer laser, however, a resin component of a chemically amplified photoresist for use in the formation of submicrometer patterns is required to have new properties unsatisfiable by conventional materials, that is, high transparency for exposure light of 220 nm or shorter and resistance to dry etching.
In the conventional lithography which uses g-beam (438 nm), i-line (365 nm) or KrF eximer laser (248 nm), a resin having an aromatic ring in its structural units, such as a novolac resin or poly(p-vinylphenol), is used as a resin component of a photoresist composition, so that the resin is allowed to exhibit resistance to etching owing to the dry etching resistance of these aromatic rings. However, a resin containing aromatic rings shows extremely strong absorption for radiation of a wavelength shorter than 220 nm. Exposure radiation is therefore mostly absorbed at a resist surface and is unable to pass to a substrate, so that no submicrometer resist pattern can be formed. Conventional resins cannot accordingly be applied to photolithography which makes use of short-wavelength radiation of 220 nm or shorter. As a consequence, there is an outstanding strong desire for a resin material which does not contain aromatic rings, has etching resistance, and is transparent to wavelengths of 220 nm and shorter.
Proposed examples of high molecular compounds having transparency to ArF eximer laser (193 nm) and dry etching resistance include a copolymer containing adamantyl methacrylate units, which is an alicyclic polymer [Takechi, et al., Journal of Photopolymer Science and Technology, 5(3), 439-446 (1992)], a copolymer containing isobornyl methacrylate units [R. D. Allen, et al., Journal of Photopolymer Science and Technology, 8(4), 623-636 (1995); ibid., 9(3), 465-474 (1996)], and a copolymer containing menthyl methacrylate units [Shida, et al., Journal of Photopolymer Science and Technology, 9(3), 457-464 (1996)].
In the above-exemplified resins, however, adamantyl-containing residue units, isobornyl-containing residue units or menthyl-containing residue units--which possess dry etching resistance--do not contain residual groups which can exhibit a difference between the solubility before exposure and that after the exposure. In addition, these alicyclic groups do not contain groups (for example, carboxyl groups) which provide the resins with solubility in an aqueous alkaline solution and also with adhesion to substrates. Accordingly, a homopolymer of a monomer containing an alicyclic group has high hydrophobicity and poor adhesion to substrates under processing (for example, silicon substrates) and can hardly form uniform coating films with good reproducibility. Moreover, due to the lack of any residual groups which make it possible to exhibit a difference between a dissolution rate before exposure and that after the exposure, no pattern can be formed by exposure. The above-described resins can hence be used as resin components in resists only when they are converted into copolymers with a comonomer capable of exhibiting a difference in solubility, such as t-butyl methacrylate or tetrahydropyranyl methacrylate, or with a comonomer capable of imparting substrate adhesion such as methacrylic acid. These comonomers have considerably low dry etching resistance and, nonetheless, their contents are required to be as high as about 50 mole %. Accordingly, such copolymers are significantly reduced in dry etching resistance and have little utility as dry-etching-resistant resins.
There is accordingly a strong desire for a new resist resin material, which has high transparency to radiation of 220 nm or shorter, possesses high etching resistance, contains functional groups permitting exhibition of a difference between the solubility before exposure and that after the exposure, permits development in an aqueous alkaline solution after exposure, and is equipped with improved substrate adhesion.
As a novel resin capable of satisfying these requirements, the present inventors already developed the resin disclosed in JP Kokai 8-259626. A further improvement in dry etching resistance is however desired.